Hereinafter, a conventional cable to waveguide transition apparatus is explained by using a coaxial cable to rectangular waveguide transition apparatus as an example of the conventional cable to waveguide transition apparatus.
FIG. 1 is a diagram illustrating a conventional cable to waveguide transition apparatus having a plane form of a backshort.
As showing, the conventional cable to waveguide transition apparatus includes a waveguide 101. The waveguide 101 further includes a backshort 102 made by a metallic conductor having a ground plane form. The backshort 102 is a predetermined distance d away from an RF probe 104 of a coaxial cable 103, where d is λg/4 and λg is one waveguide wavelength distance.
FIG. 2 is a cross sectional view taken along line A-A′ of the conventional cable to waveguide transition apparatus of FIG. 1 and FIG. 3 is a cross sectional view taken along line B-B′ of the conventional cable to waveguide transition apparatus of FIG. 1. FIGS. 2 and 3 show the waveguide 101, the backshort 102 and a location of the RF probe 104 in detail.
FIG. 4 is a diagram showing a signal path of a fundamental frequency signal of the waveguide 101 of FIG. 1 and the FIG. 5 is a diagram showing a signal path of a 2-order harmonic frequency of the waveguide 101 of FIG. 1.
As shown in FIG. 4, in case of TE10 basic mode, the fundamental frequency signal excited from the RF probe 104 to the backshort 102 is firstly reflected at the backshort 102 and then secondly reflected at a waveguide wall 109. The secondly reflected fundamental frequency signal is propagated through a wave front D which is identical to a wave front 403 of a fundamental frequency signal 402 excited from the RF probe 104 to an aperture of the waveguide 101. The secondly reflected fundamental frequency signal 404 has a phase identical to a phase of the excited fundamental frequency signal 402 at the wave front D and is propagated to the aperture of the waveguide 101 without reduction of signal. That is, the secondly reflected fundamental frequency signal 404 has same phase comparing to a phase of the excited fundament frequency signal 402 at the wave front D.
As shown in FIG. 5, the 2-order harmonic frequency signal 505 excited from the RF probe 104 to the backshort 102 is reflected at the backshort 102 and then the reflected harmonic frequency signal is propagated to the aperture of the waveguide 101. The reflected harmonic frequency signal 501 is propagated through a wave front Q which is identical to a wave front 504 of a 2-order harmonic frequency signal 503 excited from the RF probe 104 to an aperture of the waveguide 101. However, a phase of the reflected harmonic frequency signal 501 reached at the wave front Q is not identical to a phase of the excited 2-order harmonic frequency signal 503 at the wave front 504 and also, it does not generates 180° phase difference. Therefore, the reflected harmonic frequency signal 501 is propagated to the aperture of the waveguide 101 without compensation. That is, the reflected harmonic frequency signal 501 does not provide the identical phase comparing to the phase of the excited 2-order harmonic frequency signal 503 at the wave front Q and is not compensated because there is not 180° phase difference. If the reflected harmonic frequency signal 501 has the identical phase, it is propagated to the aperture of the waveguide without reduction of signal.
Therefore, if suppression of 2-order harmonic frequency signal is required, a system would be very complicated because it needs a filter having high-order harmonic frequency signal suppression characteristic.
Meanwhile, the conventional cable to waveguide transition apparatus is explained in a view of the distance d between the backshort and the probe hereinafter.
If λ represents a wavelength of a signal, λg is one wavelength of the waveguide 101 and it is expressed as:
                              λ          g                =                  λ                                    1              -                                                (                                      λ                                          2                      ⁢                      a                                                        )                                2                                                                        Eq        .                                  ⁢        1            
As shown in Eq. 1, λg is longer than λ. Therefore, d may become longer than λ/4 which is ¼ wavelength of the signal because d is λg/4 when the conventional cable to waveguide transition apparatus is designed according to it's designing method.
Occasionally, a length of the waveguide is physically limited and the distance d between the backshort and the RF probe must be designed to be shorter. In this case, a transmission characteristic of the fundamental frequency signal becomes degraded since the phase of the excited fundamental frequency signal 402 and the phase of the secondly reflected fundamental frequency signal 404 become different at the wave front D.